1. Field of the Invention
The present invention relates to an inorganic material used as a negative electrode active material for lithium ion secondary batteries, and to a method for producing the inorganic material.
2. Related Background Art
In recent years, various types of lithium ion secondary batteries have been developed. Conventionally, carbon materials have been mainly used as negative electrode active materials for lithium ion secondary batteries. However, lithium-titanium composite oxide materials have been newly developed, and are drawing attention. For example, a lithium ion secondary battery that uses LiCoO2 as a positive electrode active material and that uses Li4Ti5O12 as a negative electrode active material has been already put into practical use.
Li4Ti5O12 is a material having a spinel type crystal structure, and is capable of repeatedly absorbing or releasing Li. Therefore, Li4Ti5O12 can be used as an active material for a lithium ion secondary battery. Li4Ti5O12 absorbs or releases Li at a potential of about 1.5 V when the standard oxidation-reduction potential of lithium (Li/Li+) is defined as a reference. Accordingly, it can be thought that using Li4Ti5O12 as a negative electrode active material for a lithium ion secondary battery can realize a lithium ion secondary battery of high safety that is unlikely to cause precipitation of lithium metal at the negative electrode even when reaction overpotential occurs such as due to quick charge. Furthermore, Li4Ti5O12 has good cycle characteristics since the lattice expansion associated with charge and discharge is very small.
Materials having various characteristics improved by adding an additive element to Li4Ti5O12 have also been developed. For example, JP 2001-185141 A discloses that Li4/3Ti5/3-xFexO4 (0<x≦0.2) in which part of Ti elements of Li4Ti5O12 is substituted with Fe has an improved electron conductivity compared to Li4Ti5O12.